Method and system for controlling the photolithography process

ABSTRACT

A method and a system are presented for automatic optical control of at least one working parameter of a processing tool to be applied to a working area of a workpiece for providing certain process results. The at least one working parameter of the processing tool affects at least one parameter of the workpiece under processing. The processing tool has a preset value of the at least one working parameter prior to the processing of the workpiece. A measuring tool is applied to the workpiece prior to its processing by the processing tool for measuring the at least one parameter of the workpiece and generating measured data representative thereof. The measured data is analyzed with respect to the preset value of the working parameter and to the process results, so as to determine whether the preset value should be corrected for providing the certain process results when applying the processing tool to said workpiece. Upon detecting that the preset value should be corrected, calculating a correction value.

FIELD OF THE INVENTION

The present invention is in the field of measuring/inspecting techniquesand relates to a method and a system for controlling the operation of aprocessing toot for processing workpieces. The invention is particularlyuseful in the manufacturing of semiconductor devices to control theoperation of photolithography tools to optimize the entirephotolithography process.

BACKGROUND OF THE INVENTION

The manufacture of semiconductor devices consists of several proceduresapplied to a semiconductor wafer to define active and passive elements.The wafer is prepared and one or n)ore layers are deposited thereon.Thereafter, the process of photolithography is performed in which thesurface of a wafer with a pattern conforming to circuit elements isformed. An etching process applied to the uppermost layer follows thephotolithography, By desirably repeating these processes, a multi-levelsemiconductor wafer is produced. Thus, photolithography is one of themain steps in the manufacture of semiconductor devices. It actuallyconsists of the optical image transfer of a pattern form a mask to asemiconductor wafer.

It is a common goal of the semiconductor industry to minimize featureson, a wafer, namely to make the pattern finer and finer. Owing to thefact that optical systems used for image transfer reach theirlimitations. die lithography process should meet higher requirements ofits operational performance. This means finer process control, as wellas the development of new lithography equipment and chemicals. The majorsteps of the photolithography process are as follows:

(a) coating a wafer with a photoresist material (PR);

(b) exposing the PR to UV radiation through a mask in order to produce alatent image of the mask on the PR;

(c) developing the exposed FR in order to produce the image; and

(d) measuring and inspecting the wafer.

During the exposure of PR to UV light the PR becomes more or lesssoluble in a developing solvent, as compared to the exposed PR, therebyproducing a positive or a negative tune image, respectively.

FIG. 1 illustrates a common photolithography tools arrangement, aso-called, “link arrangement”, generally designated 1, for carrying outthe photolithography process. The main idea underlying theimplementation of such a link arrangement is that each tool is dedicatedto serve the next one in the series, so as to minimize process/toolvariations. The link arrangement 1 is composed of two main parts: aphototrack 2 and an exposure tool 3. The phototrack 2 is formed by acoater track 4 and a developer track 5, associated with cassetteload/unload stations, designated 4 a and 5 a, respectively. A robot (notshown) loads the wafer from the cassette station 4 a to the coater track4, arid, when the coating procedure is complete, transfers it to theexposure tool 3. Here, the pattern on a mask is aligned with a structurealready on the wafer (registration) by an optical means installed insidethe exposure tool 3, and the wafer is exposed to electromagneticradiation through the mask. After exposure, the robot transfers thewafer to the developer track 5 and then to the cassette station 5 a.Additionally, several different baking procedures are implemented duringthe steps (a)-(C). The coater track 2, exposure tool 3 and developertrack 5 eve tightly joined together in order to minimize processvariability and any potential risk of contamination duringphotolithography which is a very sensitive process.

The measurement/inspection step is carried out with a metrology tool 7,which is typically a big, stand-alone machine, that serves for theserial critical dimensions (CD) measurement. CD metrology tool 7measures the width of representative lines, spaces and line/space pairson the wafer. The operation of a conventional CD metrology tool is basedon two main methods: scanning electron microscope (CD SEM) and atomicforced microscope (CD AFM). CD measurements typically talk place afterthe developing step. To this end, “developed” wafers are taken out ofthe link arrangement 1 and transferred to the separate CD stationoccupied by the tool 7. Data obtained during the CD measurements isanalyzed with a processor 8 (which is typically integral with the CDmetrology tool), and then a some sort of feedback is provided (e.g. analarm in case of a width out of the permitted range) and transmitted toa relevant unit in the production line.

The quality of the entire photolithography process is defined by acombination of tolerances for all relevant parameters that can influencethe final image transfer. The main parameter that should be controlled(and the easier to be adjusted and compensated) is the exposure dosage,i.e. the amount of energy reaching the PR.

According to one known technique, so-called “send ahead wafer”, a pilotwafer is sent through the arrangement 1, namely through thecoating-exposure-developing steps, applying a certain recommendedexposure dose (and time), and then undergoes CD measurements. Theresults of the measurements will be the basis for set-up conditions ofthe entire lot, or for a correction signal to be applied to the tool 3prior to the exposure of another wafer in the lot, i.e. a feedback loop.The whole sequence of such a “send ahead wafer” procedure can take manyhours, during which valuable time of the production tools is not fullyutilized and the wafers' flow is delayed. According to anothertechnique, each lot is the basis for the next lot to run in this processrepresenting a so-called “lot-to-lot control”. By considering theresults of the previous lot, a small correction can be made. However, acertain increment in the risk exists, because the entire lot may belost. Both of these techniques are time, labor and materials consumingand usually do not reveal any problematic root.

It is known that the photolithography provides sufficient results atcertain levels of PR bleaching. Unfortunately, owing to the fluctuationsof scan speed and light intensity, it is very difficult to reproduceeach time the optimum exposure dose.

The most popular method used in production for providing a measurementdirectly correlated with the photoresist lithography image is aso-called “optimal exposure test”. According to this method, a wafercoated with a photoresist material is exposed through a mask using asequence of different dosages. Following the exposure and developmentsteps, the dose is estimated as a function of line width, utilizing theelectron microscopy technique. Notwithstanding that this methodconsiders all the relevant operations and materials of the entirephotolithography process (i.e. coat, expose, develop, bakes, resist.etc.), it consumes expensive useful time of the exposure equipment.

U.S. Pat. No. 5,620,818 discloses a photolithographic dose determinationtechnique, which utilizes diffraction of a latent image grating forconstructing a calibration curve. This technique is not compatible withon-line production control, because of the following features. Itrequires that a special mask be designed and a special test wafer,having all the relevant stack layers, be created. A large area of a teststructure is needed to provide a sufficient signal-to-noise ratio.Additionally, to consider each layer and each resist when constructingthe calibration curve, a sequence of gradual exposures of the mask onthe wafer should be conducted.

U.S. Pat. No. 5,635,285 discloses several methods of determining thecorrection for exposure. One of them is based on an exposure with aphase shift mask, which suffers from the need of an additional alignmentprocedure. Another method uses the known FLEX technique for exposures inseveral focus conditions to overcome the limits of depth of focus (DOF).This method has alignment and magnification error related problems. Yetanother method is based on the use of an additional “out of focusillumination”. More specifically, additional radiation is added outsidethe depth of focus and the mask operates as a gray scale regime.Consequently, the method is “mask regime dependent”, and thereforeshould be applied for each mask area, each layer and each productseparately.

U.S. Pat. No. 4,474,864 discloses a method for dose calculationpresenting an initial calculation procedure that relates to theconstruction of calibration curves for the first exposure set up. Thiscalibration is implemented by the coating and gradual exposure of a fewtransparent wafers for measuring the absorption resulting from bleachingat a certain single wavelength. However, the method suits a laboratorymeasurement procedure and not a real time process control since it istime consuming, and requires a long preparation procedure. This methoddoes not consider any simultaneous measurement of thickness andrefractive index (only absorption) which may vary during exposure,thereby affecting the absorption. Moreover, this method is based on theassumption that the reflection is negligible, which may actually yieldan error. According to this patent disclosure, the deduction of acalibration curve is based on a single wavelength. This indicates thatthe measurement values have no statistical averaging that can decreasethe error of the measurement itself.

In view of the above, it is evident that existing techniques forexposure dose determination/correction cannot be used as on-linemanufacturing steps, and fail to provide high accuracy and automaticanalysis and “feed-forward” dose control, rather than “feed-back”. Theexisting methods lead to the waste of wafers and other materials likephotoresist and solvents, as well as the waste of costly/useful time ofthe photolithographic tool. Hence, they reduce the production rate (i.e.throughput) of the lithographic tools. Additionally, current methods donot allow for accurate and fast determination of the optical parametersof the PR layer, such as an absorption coefficient k and a refractionindex n, and therefore do not allow for the direct dosage correction.

SUMMARY OF THE INVENTION

There is accordingly a need in the art to improve the quality of aphotolithography process used in the manufacture of semiconductordevices by providing a novel measuring method and system.

It is a major object of the present invention to provide such a methodand a system that can be used for controlling parameters of thephotolithography tools in production.

There is provided, according to one aspect of the invention, a methodfor automatic optical control of at least one working parameter of aprocessing tool to be applied to a working area of a workpiece forproviding certain process results, said at least one working parameterof the processing tool affecting at least one parameter of the workpieceunder processing, wherein the processing tool has a preset value of saidat least one working parameter prior to the processing of the workpiece,the method comprising the steps of:

(a) providing a measuring tool to be applied to the workpiece prior toits processing by the processing tool;

(b) applying the measuring tool to the workpiece for:

(c) measuring said at least one parameter of the workpiece andgenerating measured data representative thereof;

(d) analyzing said measured data with respect to said preset value ofthe working parameter and to said process results and determiningwhether said preset value should be corrected for providing said certainprocess results when applying the processing tool to said workpiece; and

(e) upon detecting that said preset value should be corrected,calculating a correction value and generating data representativethereof.

The main idea of the present invention consists of the following. Aworkpiece progressing along a production line is to be processed by aprocessing tool. The working parameter of the processing tool istypically tuned to a preset value. During the processing of theworkpiece with the processing tool, the value of this working parameteraffects some parameters of the workpiece. The processing is expected toprovide certain desired values of these workpieces' parameters(constituting the process results). However, owing to the fact thevarious procedures were applied to the workpiece before it arrives tothe processing tool (which is usually the case considering such aworkpiece as a semiconductor wafer progressing on the production line),these procedures may unpredictably influence on the parameters of theworkpiece. Consequently, the preset value of the working parameter needsto be corrected so as to meet the requirements of the realbefore-processing conditions of the specific workpiece and to satisfythe process results. For the purpose, a novel controlling method isproposed. The method consists of measuring the workpiece's parametersbefore the processing, and analyzing the same, as well as the presetvalue of the working parameter and the process results, to determine acorrection value to be applied to the preset value for achieving theprocess results when applying the processing tool to die measuredworkpiece. This technique of measuring the operational workpiece beforeits processing and adjusting the processing tool parameter accordinglyrepresents a feed forward closed loop.

Certain reference data provided and used for performing the measurementsand analysis of the measured data. The reference data is representativeof at least one calibration curve in the form, of the at least oneparameter of the workpieces as a function of the at least one workingparameter of the processing tool. The reference data also comprises anoptical model based on nominal values of certain features of theworkpiece for obtaining theoretical data representative of the at leastone parameter of the workpiece. The optical model presents theoreticaldata (mathematical equation) in the form of the intensity radiation as afunction of wavelength, wherein the radiation is that returned(reflected) from an illuminated area of the workpiece. The reflectedradiation depends on the required parameters of the workpiece inaccordance with known physical effects relevant to the specific knownkind of workpieces.

If the workpiece to be processed follows a preceding (already processed)workpiece of the same group, (e.g. one or more lot or batches in thecase of wafers) the calibration curves are known (already obtained).When dealing with a “new” group of workpieces of the known kind, thecalibration curves are prepared with respect to a first-coming workpiecein the group. In order to prepare the at least one calibration curve, aso-called “set-up operations stage” should be performed. This stageconsists of applying a desired number of “test cycles” to theoperational workpiece within a “test area” thereof. Such a workpiece aswafer is typically formed with a test area located out of the working(patterned) area and having features similar to the features of theworking area.

The test cycle consists of before-process measurement, test process andafter-process measurement steps, sequentially applied to the test areaand being carried out by the measuring tool located as described above.To this end, the measuring tool is adapted for processing the workpiecesimilar to the processing tool, a ratio between the working parametersof the measuring and processing tools being of a predetermined value.The desired number of such test cycles are performed by small movementsto corresponding number of test sites (portions) within the test areausing different values of the working parameter of the measuring toolfor each cycles and each time determining the values of the requiredparameters of the workpiece.

From the calibration curve a recommended value of the working parametercan be determined. It should be noted that the recommended value mightbe given by a manufacturer. In this case, the calibration curve servesfor determining whether this given value satisfies the process results,and, upon detecting that it does not satisfy the process results, forcalculating the correction value to be applied to the recommended value.Additionally, during the preparation of the calibration curves, thenominal values of some features of the workpiece could be updated andthe optical model so optimized could be further used for measurements.

Each measurement is performed by illuminating at least a portion (testsite) within the test area by a predetermined incident radiationspectrum and detecting radiation returned (reflected) from theilluminated area. Measured data so obtained is in the form of theradiation intensity as a function of wavelength. Using a fittingprocedure between the measured and theoretical data, the requiredparameters can be determined and analyzed to generate datarepresentative of the correction value. This data may be “fed forward”to the processing tool to adjust the value of its working parameter forobtaining the process results for this specific measured workpiece.

Preferably, the workpieces are wafers, the production line being aconventional photolithography arrangement. The working area of the waferis an area, which is formed or is to be formed with a desired pattern.The processing tool to be controlled is, preferably. an exposure tool,the working parameter to be connected being the exposure dose. However,in general, the processing tool may be any one of those used in thephotolithography arrangement (i.e. coater, developer, etc.). The atleast one measured parameter of the workpiece is the wafer'sreflectivity (i.e. reflectivity of either a substrate or a photoresistlayer on the substrate), PR refraction index, absorption coefficient orthickness.

Thus, according to another aspect of the present invention there isprovided a method for automatic optical control of at least one workingparameter of a processing too to be applied to a working area of aworkpiece for providing certain process results, wherein said processingtool is a part of a photolithography tools arrangement, said at leastone working parameter of the processing tool afffects at least oneparameter of the workpiece under processing, the processing tool has apreset value of said at least one working parameter prior to theprocessing, the method comprising the steps of:

providing a measuring tool to be applied to said workpiece prior to itsprocessing by the processing tool;

applying the measuring tool to said wafer for:

measuring said at least one parameter of the wafer and generatingmeasured data representative thereof;

analyzing said measured data with respect to said preset value of theworking parameter and to said process results for determining whethersaid preset value should be corrected for providing said process resultswhen applying the processing tool to said wafer; and

upon detecting that said preset value should be corrected, calculating acorrection value and generating data representative thereof.

According to yet another aspect of the present invention, there isprovided a measuring tool for an automatic optical control of at leastone working parameter of a processing tool which is to be applied to aworkpiece for processing a working area thereof for providing certainprocess results, said working parameter affecting at least one parameterof the workpiece under processing, wherein the processing tool has apreset value of said at least one working parameter prior to theprocessing, the tool comprising:

(1) a processing channel adapted for processing the workpiece similar tothe processing of the processing tool, a ratio between the workingparameter of the processing channel and processing tool being of apredetermined value;

(2) a measurement channel adapted for measuring said at least oneparameter of the workpiece and generating measured data representativethereof;

(3) an actuator associated with said processing channel and saidmeasuring channel for selectively actuating one of them; and

(4) a processor coupled to said measurement channel, the processor beingresponsive to said measured data for determining and analyzing said atleast one parameter of the workpiece, and calculating a correction valueto be applied to the working parameter of the processing tool prior tothe processing of the workpiece, so as to obtain said process resultswhen applying the processing tool to said workpiece.

According to yet another aspect of the present invention, there isprovided a production line having at least one processing tool adaptedfor processing successive workpieces progressing along the productionline so as to provide certain process results, wherein said processingtool has a at least one working parameter thereof that affects at leastone parameter of the workpiece under processing, the processing toolhaving a preset value of said at least one working parameter prior tothe processing of said workpieces, the production line comprising ameasuring tool installed so as to be applied to an operational workpieceprior to the processing thereof by the processing tool, the measuringtool being adapted for measuring said at least one parameter of theworkpiece and determining whether said preset value should be correctedfor providing the process results when applying the processing tool tosaid operational workpiece.

More specifically, the present invention is used with a photolithographytools arrangement for controlling the exposure tool parameter and istherefore described below with respect to this application.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a block diagram of a common arrangement of thephotolithography tools;

FIG. 2a is a block diagram of tile main components of a system accordingto one embodiment of the invention;

FIG. 2b is a block diagram of the main components of a system accordingto another embodiment of the invention;

FIG. 3 schematically illustrates the main components of a measuring toolsuitable for the system of either of FIG. 2a or FIG. 2b;

FIGS. 4a and 4 b graphically illustrate calibration curves that could beobtained with the measuring tool of FIG. 3;

FIG. 5 shows a flow diagram of a method for obtaining the calibrationcurves of FIGS. 4a and 4 b;

FIG. 6 graphically illustrates the main principles of a fittingprocedure in the method of FIGS. 5a-5 c;

FIG. 7 shows a flow diagram of a dose correction method according to oneembodiment of the invention:

FIGS. 8a-8 b illustrate flow diagrams of a dose correction methodaccording to some embodiments of the invention;

FIGS. 9a and 9 b are flow diagrams illustrating the main principles oftwo more embodiments of the invention, respectively.

DETAILED DESCRIPTION OF THE PRIOR ART AND PREFERRED EMBODIMENTS PriorArt

FIG. 1 illustrates a conventional photolithography tools arrangementwith a wafer flow chart, generally designated 1, comprising coating,exposing and developing tools 4, 3 and 5, respectively, and a robot,which is not specifically shown in FIG. 1. The arrangement 1 isassociated with the separate measurement/inspection station occupied byCD measurement tool 7 that is typically equipped by a suitable processor8 to provide a manual feedback loop for controlling the process andtools parameters. The CD tool 7 is an expensive “stand alone” devicewhich is typically manually loaded/unloaded.

Working Environment of the Present Invention

Link Arrangement

Referring to FIG. 2a, there is illustrated a system, generallydesignated 10, constructed and operated according to the presentinvention. The system 10 presents a photolithography link arrangementwith a wafer flow chart generally similar to the conventionalarrangement 1, but has some unique features consisting in the provisionof a “feed forward” loop. Same reference numbers are used foridentifying those components, which are identical in the arrangement 1and system 10, so as to facilitate the understanding of the main conceptof the invention. Thus, a phototrack 12 of the system 10, in addition tothe coating, exposing and developing tools, comprises a measuring tool14 associated with a control unit 16 and the operator's station 18. Thecontrol unit 16 is a processor equipped with a suitable image processingutility. The construction and operation of the station 18 do not form apart of the present invention and therefore need not be specificallydescribed, except to note that the station 18 typically includes apersonal computer equipped with a data base and a user interface.

Cluster Arrangement

FIG. 2b illustrates a system 100 which presents a photolithographyarrangement, so-called “cluster tool”, having a somewhat differentconstruction in comparison to that of the link arrangement 10.Similarly, same reference numbers are used to indicate those componentsthat are identical in the systems 10 and 100. Here, a robot 6 isillustrated that transmits each workpiece within the cluster tool 100.Thus, the system 100, in addition to the conventional cluster toolcomprises the measuring tool 14.

The Concept of the Measuring Tool

The measuring tool 14 is installed in a manner to be applied to anoperational workpiece prior to its arrival to the processing tool to becontrolled, e.g. the exposure tool 3. According to the example of FIG.2a, the measuring tool 14 is installed upstream of the exposure tool 3.In other words, a wafer (not shown) is brought to the measuring tool 14(by a robot) after PR coating and baking procedures. The wafer, whenarriving to the measuring tool 14, is in its ready-to-exposure positionbeing coated by a certain PR layer. The wafer is on its way to theexposure tool 3, where it should be exposed to a certain dose d₀ ofcertain UV radiation wavelength λ₀. The dose and wavelength values d₀and λ₀ are nominal, tuned (“known”) parameters of the exposure tool 3.Further “known” parameters are the substrate reflectivity R_(sub) andthose of the PR layer to be obtained after the exposure procedure withthe tool 3, i.e. of the PR layer on the processed wafer ensuing from theexposure tool 3 and progressing to the developer tool 5. The PR layerparameters are absorption coefficient k and refraction index n (opticalparameters), and thickness h. Thus, the exposure tool 3 is prepared(tuned) for the exposure procedure (i.e. exposure dose d₀) to be appliedto expected substrate material and PR layer (i.e. having certainexpected optical parameters and thickness), so as to provide thedesired, after-exposure values of the following parameters: thesubstrate reflectivity (i.e. underneath the PR layer) as a function ofthe wavelength of incident light, R_(sub)(λ); k(λ); n(λ) and h. Thedesired values of these parameters constitute the expected processresults.

However, the wafer W under processing has certain real values of theabove parameters, which values may not match the expected ones, owing tothe incoming substrate, PR materials and coating tools variations. Themeasuring tool 14 together with) the processor 16 serve for a real timedose correction Δd to be made to the exposure dose d₀ of the exposuretool 3 so as to obtain the desired PR layer parameters, when applied tothe real wafer under processing. This real time correction isimplemented with the tool 14 before the exposure procedure within thetool 3, presenting thereby a forward control of the exposure process.

The Apparatus

Turning now to FIG. 3, the main components of the measuring tool 14 aremore specifically illustrated. The tool 14 is associated with a wafer Wprogressing along the system 10 to be processed by the exposure tool.The wafer W is located on a stage, which is not specifically shown here,to progressively support the wafer during the measurement. The tool 14comprises three operational channels, generally at 20 a, 20 b and 20 c,serving, respectively, for exposure, measurement and image acquisition.The measurement channel 20 b is a spectrophotometer determining theoptical parameters and thickness of the PR layer on the wafer W.Generally, the spectrophotometric and imaging channels 20 b and 20 c maybe of any known kind, for example such as that disclosed in U.S. Pat.No. 5,517,312 assigned to the assignee of the present application.

The exposure channel 20 a is composed of an illumination unit 22, alight directing unit 24 and a detector unit 26. The illumination unit 22typically includes a light source 28 for generating UV incidentradiation, a monochromatic filter 30 and a pinhole 32. The exposureprocedure with the channel 20 a utilizes the same wavelength λ₀ as theexposure tool 3. For that reason, the monochromatic filter 30 isappropriately provided. The light directing unit 24 includes beamsplitters 34 and 36, a tube lens 38, a beam splitter 40 and an objectivelens 42. The latter is driven by a suitable motor, which is notspecifically shown, for auto-focusing purposes in a conventional manner.The beam splitter 40 deflects the part of incident radiation to thedetector 26 and the other part to the objective lens 42 to be focusedonto the wafer W (i.e. its uppermost PR layer). The operationalprinciples of all these optical elements are known per se.

The measurement channel 20 b comprises a light source 44 generatingincident light, an optical arrangement 46 and a detector unit 47. Itshould be noted that the light source 44 may be an external radiationsource with respect to the entire tool 14, provided the source 44 isassociated with a suitable light directing optics for directing lightinto the tool 14. The optical arrangement 46 operates in both thepattern recognition and measurement modes. The optical arrangement 46includes a filter 48, a grid array 50, the light directing unit 24, andan additional beam splitter 52. The filter 48 is designed to cut off theUV radiation, and is appropriately driven by an actuator (not shown) tobe displaceable between its operational and non-operational positions,being, respectively in and out of the optical path of light passingthrough the channel 20 b. The operational and non-operational positionsof the filter 48 correspond to the pattern recognition and measurementmodes of the optical arrangement 46, respectively. The detector unit 47comprises a spectrophotometer sensor 54 and a pinhole 56. The grid array50 typically facilitates the image focusing. The pinhole 56 is typicallyemployed for eliminating the exposure area edge effects. It should benoted that the objective lens 42, together with the beam splitter 40 anddetector unit 26, are preferably mounted on a movable optical head (notshown) for movement along the X, Y, Z axes. In this case, light beamsare directed to and from the optical head by means of additionalmirrors. This technique is disclosed in the above indicated U.S. Pat.No. 5,517,312.

The imaging channel 20 c is formed by the path of the spectrophotometerchannel between the wafer W and the splitting mirror 52, and by anoptical mirror 58 and a CCD camera 60. The CCD camera 60 receives thefall image including the projected grids for autofocusing purposes.

Further provided are shutters 62 and 64 (constituting an actuator)installed, respectively. in the exposure channel 20 a and measuringchannel 20 b. The shutters so provided allow for selectively actuatingeider the exposure or measurement channel. Consequently, the imagingchannel 20 c is involved, once the measurement channel 20 b is actuated.

The Measuring Tool Operation

The main mode of the tool operation includes the firstspectrophotometric measurement of a predetermined site of the PR layer,probe exposure of that site with a dose and wavelength equivalent to theexposure tool dose and wavelength the second spectrophotometricmeasurement of the same site, and then the calculation of the exposuredose correction.

Probe Exposure

The exposure channel 20 a performs a probe exposure procedure, which isgenerally similar to that of the exposure tool 3 using the samewavelength λ₀. The probe exposure, however, is carried out on test sites(not shown) typically located outside the patterned area (die area) ofthe wafer W, for example within a scribe line, and uses a certainpredetermined exposure dose d_(i) equivalent to the nominal (known) dosed₀ of the exposure tool 3.

The probe exposure dose d_(i) does not have to be the exact nominalexposure tool dose d₀ for certain mask or product, but the ratio betweenthem should be set. This is implemented either by carrying out acorrelation to CD results for the first time, or by using a known dataon the nominal exposure. Thus, the measuring tool 14 uses theproportionality ratio between the probe exposure dose d_(i) and theexposure tool dose d₀ to determine the exact correction Δd to be appliedto the exposure tool dose d₀ for proper exposure of the real wafer underprocessing.

The test site should be a clear area similar to that in the die areathat has the same stack of the resist and the underlying layers. Thesite area must be inside a clear mask area of a mask used in theexposure tool 3. Practically, the wafer area of about 50×50 micronsatisfies the mask-related and signal-to-noise requirements.

Spectral Reflectance Measurement

During the operation of the channel 20 b, the incident light passesthrough the light directing optics 46 and impinges onto the wafer W,producing reflected light that propagates in a backward directionthrough the light directing optics 24 up to the beam splitter 34. Lightpropagation is shown here schematically so as to facilitate theillustration of the main components and operational principles of themeasuring unit 14. The latter directs the reflected light towards thedetector unit 47. The beam splitter 52 transmits the pat of thereflected light to the spectrophotometric sensor 54, and reflects theother part of the reflected light to the imaging channel 20 c. Theoperation of the spectrophotometric channel 20 b is aimed at determiningthe optical parameters (absorption coefficient k and refraction indexn), substrate reflectivity R_(sub) and thickness h of the PR layer onthe wafer W. The spectrophotometric sensor 54 receives the lightreflected from the PR layer and generates measured data representativethereof. The measured data is in the form of the substrate or/and PRreflectivity I_(m) as a function of the light wavelength λ, i.e.I_(m)(λ).

Measured Data Analysis

The processor 16 is responsive to the measured data I_(m)(λ) forcalculating the above parameters R_(sub), k, n and h. To this end, theprocessor 16 is preprogrammed for running a predetermined optical model(constituting reference data) based on the nominal values of certainoptical model factors. These factors are defined by known physical lawsdescribing the light reflection from a multi-layer stack of a kindsimilar to the stack under processing, and, optionally, the kinetics ofa photo-active compound (PAC) when exposed to UV radiation. The spectrumanalysis procedure will be described in more detail further below.

The exposure of a PR to UV light results in chemical changes (e.g.,different solubility) and variation of the optical properties of the PR,namely its absorption and refraction. PR materials are typicallycharacterized by bleaching effect which is a major factor affecting theoptical absorption. The kinetics of PAC exposure is typically defined byLambert's law of absorption coupled with Beer's law. This technique usesthree photoresist parameters ABC (or Dill parameters), wherein A and Bare the bleachable and non-bleachable absorption coefficients,respectively, and C is the standard exposure rate constant. Dillparameters play an important role in the CD evaluation and simulation ofthe lithography process, and can be determined by utilizing knownoffline and inconvenient technique for measuring a transmitted intensityof a resist through a photoresist and quartz substrate covered withanti-reflection coating (ARC) on its back side. In the optical model ofthe reflection from a multi-layer stack (i.e. wafer) Dill mathematicalequations could be used, being based on theoretical data representativeof the PR reflectivity as a function of wavelength, I_(th)(λ) Since Dillparameters describe the exposure effect of PR chemical and opticalproperties, these mathematical equations could be used to understand thereflectivity from the PR layer at each dose level.

According to the present invention, there is no need for prior knowledgeof the ABC parameters of the PR. One of the preferred embodiments of theinvention presents the calculation of these parameters in a fast andaccurate way from calibration curves, when applying the values ofexposure time and determining measured data of the PR reflectivity as afunction of wavelength I_(m)(λ). Then, the kinetics of the PAC exposedto UV radiation can be accurately described. The optical model usingcorrected (updated) PR parameters could be calculated, and Dillparameters ABC could be updated. This technique can also serve foroptimizing the optical model for PR characterization in a fast andin-line mode. Once the corrected ABC are determined, a routine test ofmatching the calculated theoretical data I_(th)(λ) to the measured dataI_(m)(λ) could be presented as an additional characteristic for eachmeasurement. What can actually be retrieved from a calibration curve, isan empiric technique to obtain the exact optical parameter response tothe dose radiation. Hence, the dose control via the optical parameterresponse can be directly carried out.

To achieve accurate exposure and to ensure that the repeatable dose canbe given to the resist, a good auto-focusing mechanism should beintegrated into the system 10. The requirements for focus precisionshould be better then the depth of focus of the lithographic processbeing used. This means that, for today's state of the art applications,a focus precision level of less than 1 μm is needed. For a focus targetany high contrast pattern can be used, as long as this target is closeto the exposure location and smaller then the field of view. Theauto-focusing method itself may employ any suitable known technique, forexample that disclosed in the above U.S. Pat. No. 5,604,344.

Calibration Procedure

General

The measurement unit 14 operates in the following manner. A firstoperational stage of the unit 14 consists of the creation of a databasefor the specific PR layer, which is to be currently exposed. Morespecifically, the previously provided optical model is optimized andcalibration curves (constituting reference data) are prepared based onthe real PR material, substrate reflectivity R_(sub) and nominalthickness. It is assumed that the effect of a possible change in thenominal values of h and R_(sub) is supposed to be small so as not tocause substantial changes in the calibration curve.

The calibration curves present the probe exposure dose variationsaffecting the optical parameters k and n. FIGS. 4a and 4 b illustratefour graphs C₁ and C₂, and G₁ and G₂ representing, respectively, the PRoptical parameters as functions of the exposure dose, i.e. k(d_(i)) andn(d_(i)), and the exposure dose changes with the changes of the opticalparameters as functions of dose, i.e. ∂d/∂k(d) and ∂d/∂n(d).

If changes in thickness and substrate reflectivity are detected (i.e.measured), one can calculate the compensation that is needed for theexposure dose by using the ratio of the reflection with the currentvalues of thickness and substrate reflectivity to the known reflectionfrom the nominal values of thickness and substrate reflectivity.

Calibration Curves Determination

FIG. 5 illustrates the main steps of the first operational stage, i.e.set-up operations. Initially, a so-called “pre-alignment” (orregistration) procedure is performed in a conventional manner beingapplied to a first coming wafer from the wafer stack to be processed,i.e. a pilot wafer (step 70). lien, the pilot wafer (or correspondingoptics of the measurement channel 20 b) is moved to a predetermined testsite position, so-called “calibration site” (step 72). Here, the sitepattern recognition and auto-focus correction procedures are implementedwith the imaging channel 20 c (step 74). For the purpose, the shutters62 and 64 are in their non-operative and operative positions,respectively, so as to block the exposure channel 20 a and actuate themeasurement channel 20 b. As to the filter 48, it is in its operationalposition, namely in the optical path of light propagating through thechannel 20 b. Thereafter, a certain exposure-spectrophotometric schemeis applied (step 76). This procedure consists of the following.

The filter 48 is shifted into its non-operational position, being out ofthe optical path of light passing through the channel 20 b. In otherwords. the channel 20 b is switched to the measurement inode. The PRreflectivity as a function of wavelength, i.e. I_(m)(λ), is measured andoptical parameters R_(sub)(λ), n(λ), k(λ) and thickness h of the PRlayer are calculated, using the optical model optionally based on thenominal values of Dill parameters. Then, the shutters 62 and 64,respectively, open and close the exposure and measurement channels 20 aand 20 b, and the probe exposure is applied using a predeterminedexposure dose d_(i) proportional to the exposure tool nominal dose d₀.Thereafter, the shutters 62 and 64 are operated to. respectively, blockdie exposure channel 20 a and actuate the measurement channel 20 b, and“after-exposure” measurement is carried out to obtain measured dataI′_(m)(λ) and to calculate the parameters k′_(i)(λ), n′_(i)(λ),thickness h′_(i) and reflectivity R′_(sub). The “calibration site”actually comprises an array of m test sites, By movements (which areoptionally small) from site to site within this array and executing theabove scheme at m different exposure doses. the calibration curves k(d)and n(d) are obtained (step 78). From the calibration curves, arecommended exposure dose d_(rec) is determined (step 80). Therecommended dose d_(rec) is a dose value at which each of the calculatedparameters k, n, R_(sub) and h is equivalent to a corresponding one ofthe desired parameters to be obtained by the exposure procedure with theexposure tool 3. Thus, the data base associated with this specific PRcoating is provided (step 82). This data base comprises the PR opticalparameters and thickness as functions of dose (i.e. k(d); n(d); h(d))with respect to a certain substrate reflectivity R_(sub). It should benoted that, if a certain nominal dose is given by end user, and theabove steps associated with the determination of the recommended dosecould be eliminated. In this case, the nominal dose value serves as tilerecommended dose to be analyzed through measurements and corrected, ifrequired.

At this stage, namely for this recommended dose value d_(rec) of theexposure tool 3, the optical model could be optimized by performing afitting procedure between the measured data I′_(m)(λ) and thecorresponding theoretical data I_(th)(λ). As indicated above, thetheoretical data is obtained using certain optical model factors. Thesefactors are adjustable to satisfy a certain condition, which istypically in. the form of a merit function defining a so-called“goodness of fit” between the measured data and theoretical data.

Measurement Procedure and Exposure Control

The measurement procedure consists of the determination of all theparameters affecting the exposure dose, namely: the substratereflectance, PR thickness and PR optical parameters. FIG. 6 illustratestwo graphs I_(m) and I_(th) representing the PR reflectivity as afunction of wavelength, obtained through the measurement and opticalmodel, respectively. By varying (fitting) the values of PR parameters h,n, k and R_(sub), the sufficient goodness of fit could be obtained. Bydoing this, the optical model factors are updated and the optical modeloptimized.

Measurement Procedure and Exposure Control

The substrate reflectance contribution to the exposure dose could becalculated by two basic techniques:

(1) an additional measurement of the site before the PR coating, by thespectrophotomretric channel;

(2) concurrently calculating the substrate reflectivity R_(sub) and PRparameters from the measured data I_(m).

In both cases, the effect of substrate reflectivity fluctuation from thenominal reflectivity value can be deduced and taken into consideration.

If the calculation show that the recommended dose value does not providethe process results, i.e. the desired values of the PR parameters, theoperation is moved to a further test site, so-called “measurement site”,and a further operational stage is carried out for determining thecorrection which is to be applied to the recommended exposure dosed_(rec). The main steps of this operational stage are illustrated in aself-explanatory manner in FIG. 7.

As described above, the set-up operations result in the provision of adatabase associated with this specific PR coating is provided (step 82).Thereafter, the measure-exposure-measure scheme is performed one timeusing the recommended exposure dose d_(rec) and the exposure toolwavelength λ₀ (step 92). Each of the before-exposure PR parametersk₁(λ), n₁(λ), h₁ and after-exposure PR parameters k₂(λ), n₂(λ), h₂ isseparately obtained. The corresponding parameters and compared, so as todetermine the differences between them, that is Δk, Δn and Δh (step 94).This enables to detect whether or not the recommended exposure dosed_(rec) needs to be corrected (step 96). If so, the corrected exposuredose d_(cor) is calculated in the following manner:

d_(cor)=d_(rec)+Δd

Here, Δd is a function of Δh, Δk, Δn, δR_(sub), that is Δd=f(Δk, Δn, Δh,δR_(sub)) or, more specifically:${\Delta \quad d} = {{\left( {k_{c} - k_{m}} \right) \cdot \left( \frac{\partial d}{\partial k} \right)_{c}} + ɛ_{R_{sub}}}$

wherein k_(c) and k_(m) are the values of the absorption coefficientobtained from the calibration curse and through measurement,respectively; (∂d/∂k)_(c) is a slope value corresponding to the dose forthe “calibration” absorption coefficient; ε_(Rsub) is the dose changedue to the absolute reflectivity level of the substrate, including allthe layers in the stack, at the exposure wavelength. When the ratioδR_(sub)/R_(sub) satisfies the following condition:$\frac{\delta \quad R_{sub}}{R_{sub}}\quad \text{≺≺}\quad 1$

then for the dose change we have:

ε≈−α·δR_(sub)

wherein α−Const for a certain PR

It should be noted that the measurement (second) stage could includemore than one measurement-exposure-measurement cycle. In other words,the predetermined parameters could be retrieved by applying more thenone partial dose, rather than one nominal dose.

To more clearly illustrate the above-described unique features of thepresent invention, let us consider the following example for theexposure control.

EXAMPLE

In this example, δR=0, Δh=0. Turning back to FIGS. 4a and 4 b, thecalibration curves are obtained. In other words, the relevant data baseis already created as described above. The recommended exposure dose isas follows:

d_(rec)=4 mJ/cm²

The calibration curves C₁ and G₁ show that for this dose value we have:

=k_(c)=0.006417; (∂d/∂k)_(c)=−375.37

Considering the typical case that a dose correction is needed, thesecond operational stage is performed i.e. the exposure with therecommended exposure dose is applied to the measurement site, and themeasured (calculated) data for the absorption coefficient is determinedto be:

k_(m)=0.005636

As seen in the calibration curves C₁ and G₁, for the measured valuek_(m) of the absorption coefficient, we have:

d_(m)≈4.5 mJ/cm² (∂d/∂k)_(m)=−393.3

Using the above equation for the corrected dose value, we have:

Δd=½(0.006417−0.005636)·(−375.37−393.3) Δd=(−0.3)mJ/cm²

Hence, in order to obtain the desired value of the PR bleaching, theexposure dose of the exposure tool 3 should be decreased by 7.5%. Inthis specific example, the complete sequence of dose correction isimplemented with respect to k-calibration curves. The similar proceduremay be done for the n-calibration curves.

Process Control Applications

The processor 16 determines the correction value Δd and generates datarepresentative thereof. This data is transmitted to the respectiveutility of the exposure tool 3 either directly or through a special hostfacility (not shown). This technique actually represents the correctionsignal detection and “feed forward” to the exposure tool 3.

It should be specifically noted that the above-described scheme of afast accurate and automatic measurements aimed at determining the properdose for exposure tool 3 can be applied also to different kinds offeatures like test structures, patterns and specially designed marks onthe wafer.

Additionally, the above-described method of fast, accurate and automaticmeasurement could be used for the entire process control. Indeed, themethod allows for comparing and estimating the parameters of anyradiation-sensitive material, including liquid and solid ARC, on anysubstrate. The measurement could be performed across the wafer, in awafer to wafer, lot to lot or batch to batch fashion. The measuring unit14 could be installed as a part of any photolithography toolsarrangement, such as “link” or “cluster”. By means of theabove-described operation applied to each wafer, the trend between waferto wafer could be determined to give feedback corrections to the amountof exposure that is needed to overcome any fluctuations which have a“pre-exposure origin”, for example PR photosensitivity. It is known thatfinally obtained CD may be compensated by various parameters. In thisconnection, the variability of the parameters k, n, and h, that will befound during the measurements, can be compensated by these parameters.Several feedback options can be done for closing the loop and correctingfor the variability of k, n, h or R_(sub) parameters of PR.

The provision of the measuring tool 14 enables to obtain such data whoseprocessing allows for establishing an in-line or offline closed loopcontrol for coating parameters, such as spin speed, humidity,temperature, exhaust, etc. Additionally, the measuring tool could beemployed for in-line or off-line closed loop control of the developingprocess parameters, for example the develop time, post exposure bake(PEB) time, PEB temperature.

It should also be noted, that the calibration curves so obtained providefor the fast, accurate and automatic in-line determination of the ABCparameters (i.e. Dill parameters). Indeed, by a series of partial.exposures the total PR bleaching can be achieved. From the fitting tothe initial “before-exposure” reflectivity spectrum and from the fittingto the final reflectivity spectrum the initial absorption and finalabsorption could be determined, the difference between them representingthe first Dill parameter A.

The above-described technique enables the exposure dose for any locationon the wafer to be in-line predicted, using the in-line determination ofABC parameters. This allows for predicting the CD values and for in-linesimulation using the actual values for R_(sub) ABC, n, k, and hparameters, which is advantageous, as compared to any off-linesimulation.

Reference is now made to FIGS. 8a-8 b illustrating flow diagrams of someother embodiments of the process control procedures. These proceduresdiffer from each other by the process stage at which measurement isconducted.

Before Coating Measurement

The substrate reflectivity R_(sub) is measured prior to the coatingprocedure (step 103). The measured reflectivity could be used forpartial control of the exposure dose or for minimizing the freedomdegrees of further optimization, as will be described below.

Measurement-Exposure-Measurement

A wafer, after the PR coating (step 98) and soft-bake processes (step100) applied thereto, undergoes the measurement of the relevantparameters: k₀, n₀ and h₀ (step 102). Then, the aboveexposure-measurement scheme is applied either to the first site onlyusing the partial exposure dose (steps 104 and 105) or to the secondtest site using the full exposure dose (steps 106 and 107).After-exposure values of these parameters are determined, that is k₁,n₁, h₁, R₁ and k₂, n₂, h₂, R₂ (steps 108 and 110). By analyzing the“before-exposure” and “after-exposure” measurements (step 114), theexposure dose of the exposure tool 3 may be adjusted.

Calibration of the Measurement and Exposure Tools

A similar full exposure may be applied to a test site by the exposuretool 3 (step 111) for determining after-exposure values of the relevantparameters k*₂, n*₂, h*₂, R*₂ (step 112). Steps 106, 107, 110 could beapplied for determining k₂, n₂, h₂, R₂. The obtained results would becompared to the results of the steps 111 and 112. Steps 111 and 112present the similar full exposure that can be applied to the test siteby the exposure tool 3 for determining after-exposure values of therelevant parameters k*₂, n*₂, h*₂, R₂. Comparison between two differentfull exposure results could serve for matching of the exposureprocedures conducted by the exposure tool and the measuring tool.

Thickness Loss Due to the Exposure

By comparing the results of steps 102 and 110, thickness loss due toexposure could be deduced. This could serve for dose control viathickness change during exposure. The aforementioned control will beevaluated from the calibration curve h(d) that could be determined fromthe calibration procedure.

De-protection Induced Thickness Loss (DITL)

An additional analysis of the two different after-exposure measurementsassists in the estimation of a thickness loss of the wafer after the PEBprocess. Another option shown in FIGS. 8a-8 b is that of the wafer beingfurther processed within the exposure tool 3 (step 116), undergoingmeasurement (steps 111, 112) and undergoing PEB process (step 118).Thereafter, the after-PEB measurement is performed (step 119) and k′₂,n′₂, h′₂, R′₂ parameters are determined at the second test site (step120). The determined values are compared to those obtained from steps111 and 112. This enables a baseline for “after expose measurements” tobe created for the estimation of De-protection Induced Thickness Loss(DITL) in chemically amplified resists.

Procedures Without the Use of Exposure Channel

Turning now to FIGS. 9a and 9 b, two more examples of a method accordingto the invention are presented in a self-explanatory manner. Accordingto these examples, the measuring tool 14 does not need any processing(exposure) channel, but operates only for measuring the relevantparameters of the wafer prior to the processing, namely prior to thecoating and exposure procedures in the example of FIG. 9a and prior tothe exposure procedure only in the example in FIG. 9b. In these cases,the measuring tool may be a spectrophotometer of any suitable kind,capable of providing fast and accurate measurements of the intensity oflight returned from an article. For example, the construction andoperation of such a spectrophotometer may be similar to that disclosedin U.S. Pat. No. 5,517,312 assigned to the assignee of the presentapplication.

Thus, the present invention enables to obtain data that can be used toinitiate a base line for controlling the lithography process affectingthickness changes during the PR exposure or PEB, as a part of moreelaborate setup for process control. Data obtained from the above methodmay be used for the correlation of other metrology tools results, forexample construction of a calibration curve between this method to anyone of the known metrology tools for measurement of critical dimensions.(e.g. SEMCD, AFM).

In view of the above, the advantages of the present invention areself-evident. It provides a method and apparatus for determining themid-, near- or deep UV accurate exposure dose of a patterning exposuretool, for every product wafer. The invention allows for automatic, fastand accurate dose controlling of the photolithographic tools inproduction. To measuring unit 14 so designed enables the integration ofa measurement system inside a photolithographic tools arrangement inorder to obtain a closed loop feed forward control based on measuringproduction wafers. The integration of such a tool will enable, amongother things, a fast response and comparison for wafer to waferthickness variations, wafer thickness non-uniformity, reflectancevariations in the resist, ARC or layers below them. The invention allowsfor thickness measurement as a function of time to achieve the controlof thickness changes caused by volatile component evaporation or acidloss. The measuring unit 14 may include several metrology operations formonitoring several lithography steps, like exposure and PEB.

The present invention provides a so-called “information baseline for allother process and/or tools variables and can be used for compensatingany initial fluctuations. For example, it provides for controlling aphotolithography process by analyzing data obtained from measurements ofthe differences in the optical parameters and thickness between theinitial, pre-soft-bake state and final, after-exposure states, theinitial, after-exposure and final, after-post-exposure-bake states. Thesituation of after-expose state can be easily achieved with the systems10, 100 by applying the full nominal exposure of the exposure tool andafter the bake procedure that follows the exposure. The invention alsoprovides a direct method for determination of the Dill parameterswithout any need for special mask or transparent wafer, or specialoff-line tests. The present invention can be used each time thecorrelation of the dose correction to the known metrology tool resultsis required. The present invention overcomes the difficulties ofcontrolling the exposure step for photolithographic process, without anyneed for an in situ internal calibration of the exposure tools orprevious “off line” measurement of the resist parameters.

Those skilled in the art will readily appreciate that many modificationsand changes may be applied to the invention as hereinbefore exemplifiedwithout departing from its scope defined in and by the appended claims.For example, the present invention may be used for controlling otherparameters of the exposure tool, e.g. the exposure time. Alternatively,the present invention may be used for controlling the operation of otherphotolithography tools, as well as of any other processing tool forprocessing workpieces. The processing tool that could be controlled bythe invented technique should be characterized by a working parameterwhose variations affect the optically measurable parameters of theworkpiece. This enables the measuring tool to be installed upstream ofthe processing tool and provide feed forward loop for correcting theworking parameter when required.

What is claimed is:
 1. A method for automatic optical control of atleast one of exposure and focus parameters of an exposure tool in aphotolithography arrangement to be applied to a workpiece coated with aphotoresist layer, the method comprising the steps of: (a) applying ameasuring tool comprising a spectrophotometer to the workpiece prior toprocessing of the workpiece by the exposure tool; (b) measuring at leastone parameter of the workpiece and generating measured datarepresentative thereof, said at least one parameter of the workpiecebeing a parameter defining said at least one of the exposure and focusparameters of the exposure tool to enable obtaining desired parametersof the workpiece by exposing the workpiece with the exposure tool; and(c) analyzing said measured data and determining a required value ofsaid at least one of the exposure and focus parameters of the exposuretool to provide said desired parameters of the workpiece resulting fromapplying the exposure tool to said workpiece;.
 2. The method accordingto claim 1, further comprising the step of: providing reference datarepresentative of at least one calibration curve in the form of said atleast one parameter of the workpieces as a function of said at least oneof the exposure and focus parameters of the exposure tool.
 3. The methodaccording to claim 1, and also comprising the step of: providing anoptical model based on nominal values of certain features of theworkpiece for obtaining theoretical data representative of said at leastone parameter of the workpiece.
 4. The method according to claim 3, andalso comprising the step of: analyzing said theoretical and measureddata and optimizing the optical model by correcting the nominal valuesof said certain features of the workpiece.
 5. The method according toclaim 1, wherein at least one of the exposure and focus parameters ofthe exposure tool is given a preset value prior to processing of theworkpiece by the exposure tool, said method further comprising the stepof: (d) comparing the preset value with the required value and, when thecomparison indicates that a difference exists, calculating a correctionvalue and generating data representative of the correction value (e) inresponse to said data representative of the correction value, correctingsaid preset value of the working parameter of the processing tool to beapplied to the measured workpiece.
 6. The method according to claim 1,wherein the exposure tool is a part of a production line, said workpieceprogressing along the production line towards the exposure tool throughsaid measuring tool.
 7. The method according to claim 1, wherein saidphotolithography tool arrangement further includes a coating tool forprocessing the workpiece, said method further comprising the step ofapplying the measuring tool to the workpiece prior to processing of theworkpiece by the coating tool.
 8. The method according to claim 1,wherein said at least one parameter of the workpiece is an absorptioncoefficient.
 9. The method according to claim 1, wherein said at leastone parameter of the workpiece is a refraction index.
 10. The methodaccording to claim 1, wherein said at least one parameter of theworkpiece is a reflectivity.
 11. The method according to claim 1,wherein said at least one working parameter is a thickness of at leastan uppermost layer thereof.
 12. The method according to claim 1, whereinsaid workpiece is a semiconductor wafer.
 13. A method for automaticoptical control of at least one of exposure and focus parameters of anexposure tool in a photolithography tools arrangement to be applied to aworking area of a workpiece coated with a photoresist layer forproviding a desired value of at least one parameter that the workpieceis to have after having been exposed with the exposure tool, the methodcomprising the steps of: applying a measuring tool comprising aspectrophotometer to the workpiece prior to exposing the workpiece withthe exposure tool; measuring, with the measuring tool, said at least oneparameter of the workpiece and generating measured data representativethereof; said at least one parameter of the workpiece being a parameterdefining said at least one of the exposure and focus parameters of theexposure tool should have in order to cause the at least one parameterof the workpiece to be given the desired value after exposing theworkpiece with the exposure tool; and analyzing said measured data fordetermining a required value for said at least one of the exposure andfocus parameters of the exposure tool provide the desired value of theat least one parameter of the workpiece after exposing the workpiecewith the exposure tool.
 14. The method according to claim 13, wherein atleast one parameter of the workpiece is a reflectivity.
 15. The methodaccording to claim 13, wherein at least one parameter of the workpieceis a refraction index.
 16. The method according to claim 13, wherein atleast one parameter of the workpiece is an absorption coefficient. 17.The method according to claim 13, wherein said at least one parameter ofthe workpiece is a thickness of at least an uppermost layer thereof. 18.The method according to claim 13, wherein said workpiece is asemiconductor wafer progressing along a production line.
 19. Aproduction line having a photolithography tools arrangement including anexposure tool for processing successive workpieces coated with aphotoresist layer and progressing along the production line so as toprovide a desired value for at least one parameter of a workpiece afterexposing the workpiece with the exposure tool, the production linecomprising a measuring tool comprising a spectrophotometer installed soas to be applied to each workpiece prior to the processing thereof bythe exposure tool, the measuring tool being operable for measuring saidat least one parameter of the workpiece and determining a required valueof at least one of exposure and focus parameters of said exposure toolto provide the desired value resulting from exposing the workpiece withthe exposure tool, wherein said at least one parameter of the workpieceis a parameter defining the at least one of the exposure and focusparameters of the exposure tool to enable obtaining the desired valuefor the at least one parameter of a workpiece after exposing theworkpiece with the exposure tool.
 20. The production line according toclaim 19, wherein said successive workpieces are semiconductor wafers.